1. Field of the Invention
The present invention relates to a novel battery and a method for producing the same. More particularly, the present invention is concerned with a non-aqueous secondary battery comprising a positive electrode comprising a cathode active material layer, a negative electrode comprising an anode active material layer, and a porous separator which is disposed between the positive electrode and the negative electrode and which is directly formed, in an immobilized form, on at least one active material layer selected from the group consisting of the cathode active material layer and the anode active material layer, wherein the positive electrode, the negative electrode and the separator are disposed in a casing containing an electrolyte, and wherein the porous separator comprises at least one layer of an aggregate form of particles of at least one insulating substance and a binder which is mixed with the particles to thereby bind the particles together, the layer of the aggregate form of particles having a three-dimensional network of voids which function as pores in the porous separator and which are capable of passing ions therethrough. The present invention is also concerned with a method for producing the above-mentioned novel battery. The battery of the present invention is advantageous not only in that the battery exhibits excellent discharge characteristics even at a high discharge current density without sacrificing safety, but also in that the amount of active materials which can be accommodated in the battery per unit volume thereof is large, as compared to the amounts in the case of conventional batteries.
2. Prior Art
In recent years, various demands have been made on electrical appliances, wherein the electrical appliances should be reduced in size and weight, wherein they should have multifunctionality and wherein they should be codeless (portable). For meeting these demands, development of high performance batteries has been vigorously studied. Batteries can be generally classified into primary batteries which are not rechargeable and secondary batteries which are rechargeable so that they can be repeatedly used. Examples of primary batteries include manganese dioxide batteries and an alkaline manganese dioxide dry cell. With respect to these primary batteries, various improvements have been made, and the primary batteries are used in a wide variety of fields. On the other hand, examples of secondary batteries include lead storage batteries, nickel-cadmium batteries and nickel-hydrogen batteries. Recently, a commercial demand for a secondary battery, particularly a lithium ion secondary battery using a non-aqueous electrolytic liquid has been increasing, since the lithium ion secondary battery can exhibit high voltage and high capacity even in a compact and lightweight form.
The performance of the above-mentioned batteries can be improved, for example, by increasing the amount of active materials and/or the amount of electrolyte which can be accommodated in a battery per unit volume of the battery, or by improving the ion conductive property between the positive electrode and the negative electrode.
Particularly, in the case of a battery using a non-aqueous electrolytic liquid (such a battery is hereinafter, frequently, referred to simply as a "non-aqueous battery"), such as the above-mentioned lithium ion secondary battery, since the non-aqueous liquid used in such a battery has a poor ion conductive property as compared to an aqueous electrolytic liquid, it is desired to improve the ion conductive property between the positive electrode and the negative electrode. For this purpose, generally, such a battery is designed to have a construction in which a plurality of unit cells (each comprising a positive electrode, a negative electrode and a separator) are laminated, or a construction in which a unit cell is spirally wound into a spirally wound structure, so as to increase the effective area of electrodes, at which the positive electrode and the negative electrode face each other. However, a satisfactory improvement in the ion conductive property has not yet been achieved.
As an example of the most effective methods for achieving an improvement in the ion conductive property, there can be mentioned a method in which a separator having a small thickness and an excellent ion permeability is used.
As a separator used in a conventional battery, generally, use is made of a microporous film made of a polyolefin resin, such as polyethylene or polypropylene. For example, as described in Unexamined Japanese Patent Laid-Open Specification No. 3-105851, the above-mentioned microporous film can be produced by a method in which a molten mixture comprising a polyolefin resin composition is extrusion-molded into a sheet, substances other than the polyolefin resin are removed from the sheet by extraction, and the resultant sheet is subjected to stretching.
The above-mentioned resin film separator needs to have a mechanical strength such that occurrence of the breakage of the separator can be avoided during the production of a battery. Due to such a required mechanical strength, it is difficult to reduce the thickness of the separator to less than a certain thickness. Therefore, in the case of the above-mentioned non-aqueous battery (such as a lithium ion secondary battery) having a construction in which a plurality of unit cells are laminated, or a construction in which a unit cell is spirally wound into a spirally wound structure, the amount of the unit cell which can be accommodated in the battery per unit volume thereof inevitably becomes small due to the restriction in respect of reduction of the thickness of the separator. Further, even when it is attempted to improve the ion conductive property between the positive electrode and the negative electrode by increasing the porosity of the conventional resin film separator, satisfactory results cannot be obtained (see Comparative Example 1 of the present specification) (the reason for this has not yet been elucidated). The above-mentioned separator made of a resin film is also disadvantageous in that the resin film separator has poor durability. Therefore, when such a separator is used in a secondary battery, the separator is deteriorated during the repetition of the charge/discharge operations, so that the cycle characteristics of the battery become poor (see Comparative Example 3 of the present specification). Further, in a battery using a conventional separator, use must be made of a large amount of the separator which is produced by the above-mentioned cumbersome, costly method, so that the ratio of the cost for the separator to the total cost for the battery becomes relatively high. Therefore, especially in the case of the above-mentioned non-aqueous battery, such as the lithium ion secondary battery, in which the conventional resin film separator is used, a large area of separator is needed due to the above-mentioned unique construction of such a battery and the cost for the separator becomes disadvantageously high, thereby rendering high the production cost for the battery.